1. Field of the Invention
The present invention relates to an antireflection film, a polarizing plate using the antireflection film, and an image display device using the antireflection film or polarizing plate. More specifically, the present invention relates to an antireflection film enabling to prevent the visibility at a high level from worsening due to projection of reflected light, causing less reduction in the display quality, hardly allowing point defects or scratches to be perceived, and having good durability.
2. Description of the Related Art
In various image display devices such as liquid crystal display device (LCD), plasma display panel (PDP), electroluminescent display (ELD) and cathode ray tube display device (CRT), an antireflection film is disposed on the display surface so as to prevent the reduction in contrast due to reflection of outside light or projection of figure. Accordingly, the antireflection film is required to have not only high antireflection performance but also high transmittance, high physical strength (e.g., scratch resistance), chemical resistance, and weather resistance (e.g., resistance against humidity, heat or light).
In such an antireflection film, a low refractive index layer which is a thin-film layer of 200 nm or less in thickness is provided at least on the outermost surface, and reflection is prevented by the effect of optical interference of the low refractive index layer. However, in the case of a one-layer thin-film interference type of preventing reflection by one low refractive index layer which is a simplest configuration, a practically usable low refractive index material satisfying a reflectance of 0.5% or less and at the same time, having neutral color tint, high scratch resistance, chemical resistance and weather resistance is not present. In order to achieve a reflectance of 0.5% or less, there is known a multilayer thin-film interference-type antireflection film for preventing reflection by the effect of optical interference of multiple layers, such as a two-layer thin-film interference type where a high refractive index layer is formed between the transparent support and the low refractive index layer, and a three-layer thin-film interference type where a medium refractive index layer and a high refractive index layer are sequentially formed between the transparent support and the low refractive index layer. In particular, for preventing reflection in a wide wavelength range and achieving a low reflectance while suppressing the reflected color, a three-layer thin-film interference type is preferred.
As for the thin-film layer (e.g., high refractive index layer, medium refractive index layer, low refractive index layer) used in the multilayer thin-film interference-type antireflection film, a multilayer film where a transparent thin-film layer of metal oxide is stacked is heretofore widely used. The transparent thin film of metal oxide is usually formed by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method, particularly, by a vacuum vapor deposition method or a sputtering method which are a kind of the physical vapor deposition method.
However, the method of forming a transparent thin film of metal oxide by vapor deposition or sputtering is not suited for mass production because of its low productivity, and a formation method by a wet film-forming method with high productivity, particularly, a coating system, is proposed.
In the case of producing an antireflection film by a coating system, a coating composition prepared by dissolving or dispersing a film-forming composition having a specific refractive index in a solvent is coated on a substrate, dried and, if desired, cured, whereby a single-layer or multilayer thin-film layer can be formed. In the case of a single layer, it may suffice if a layer having a refractive index lower than that of the substrate (low refractive index layer) is formed to, in terms of optical layer thickness, a thickness of about ¼ of the designed wavelength. When more reduction of reflection is required, it is known to form a layer having a refractive index higher than that of the substrate (high refractive index layer) between the substrate and the layer having a low refractive index (see, for example, Japanese Patent No. 3034044).
Such a multilayer-type antireflection film can reduce the reflection, but when the thickness of each layer fluctuates, the reflected color is changed. For example, in the wet coating, although this may vary depending on the coating system, it is very difficult to form an optical interference thin film in thickness of approximately from 50 to 150 mm with good precision. Usually, thickness unevenness of 5 to 10 nm is generated and there arises a problem that reflected color corresponding to the unevenness is observed or a failure such as coating streak is liable to be highly visible. In order to solve such a problem, efforts are being made to reduce the thickness unevenness of the formed film coating by adjusting the viscosity of the coating solution or devising the coating apparatus. Furthermore, for example, JP-A-2003-121606 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses an antireflection film where the wavelength dependency of reflectance and in turn the change of color tint are reduced by restricting the thickness and refractive index of each layer to specific ranges.
However, with the refractive index of each layer described in Examples of JP-A-2003-121606, the change of reflected color due to fluctuation in the thickness of each layer is still large, and more improvement is demanded.
In other words, conventionally proposed antireflection films cannot sufficiently respond at the fluctuation in thickness of each layer in terms of satisfying both the reduction of reflection and the suppression of change in the reflected color, and development of an antireflection film satisfying both of these properties is demanded.